Aflatoxin M1 (AFM1), a metabolite of the potent carcinogenic mycotoxin aflatoxin B1 (AFB1), has been widely detected in the milk and edible tissues of animals exposed to AFB1. Evidence has shown that AFM1 is also a potent carcinogen. Limited availability of AFM1 has so far hindered the progress of the urgently needed comprehensive assessement of the food safety significance of AFM1. Our recently developed fermentation technique is now capable of producing sufficient quantities of AFM1 for detailed toxicological studies on this food-borne carcinogen. The objective of this research is to determine the carcinogenic activity of AFM1 as compared to that of AFB1 in the mammalian species of different susceptibility such as the rat, mouse, and rhesus monkey. In in vitro experiments, human liver specimens will also be used. This objective will be achieved by establishing correlations between in vivo oncogenic activity and each of the following biochemical and morphological parameters: (1) in vitro and in vivo metabolic profile, (2) covalent binding with cellular receptors such as DNA, RNA, and proteins, (3) morphological changes in the ultrastructures of target hepatocytes, and (4) clinical chemical tests that may be useful in diagnosis of aflatoxin exposure. These correlations will be compared in terms of the feasibility and validity of using these parameters as measurements or indicators of species susceptibility to the carcinogenic effect of AFM1.